1. Field of the Invention
The technical field of the invention is that of solid polymer electrolytes.
Such electrolytes are used in various electrochemical systems, in particular in rechargeable or non-rechargeable lithium electrochemical generators, but also in electrochromic systems or in super-capacitors.
They can also be used for the production of the composite electrodes of a generator.
2. Description of the Related Art
Lithium is increasingly used in batteries because of the redox potential of the Li/Li+ couple, which is situated xe2x88x923 volts with respect to the standard hydrogen electrode and which allows it to be used for high voltage generators. Furthermore, its high specific capacity allows it to be used for generators with a high specific energy. Lithium can be used in the metallic form in rechargeable or non-rechargeable generators with a negative lithium electrode and in the ionic form in generators with a negative lithiated carbon electrode, better known under the name of lithium ion batteries.
The use of solid polymer electrolytes for lithium batteries has been proposed for a long time. This is because such electrolytes make it possible to produce, with simple preparation processes, thin batteries of varied shapes. The known electrolytes are composed either of a salt dissolved in a dry polymer or of a polymer swollen or gelled by a solution of a salt in a mixture of polar aprotic organic solvents.
The known polymer matrixes can be produced based on thermoplastic materials, such as PVC (polyfvinyl chloride)), PAN (polyacrylonitrile) or PVDF (poly(vinylidene fluoride)). In the absence of solvents (or plasticizers), the mixtures between these thermoplastics and the salts form insulators or very poor ionic conductors.
After addition of solvents, they become sufficiently conducting at ambient temperatures or at temperatures below ambient temperature.
However, PVC is unstable in basic medium and cannot therefore be used in combination with lithium metal or with a lithium ion system.
Likewise, the fluorine atoms of PVDF are reactive with respect to metallic lithium. Furthermore, it should be noted that the CH2xe2x80x94CF2 repeat unit of PVDF has no particular affinity with lithium salts (weak solvating nature for the Li+ cation), which compromises its electrochemical performance.
From the viewpoint of the preparation processes, solvents with low boiling points ( less than 70xc2x0 C.) which make possible the preparation of stable solutions are not known for PVDF, which complicates the industrial processing.
PVCs and PVDFs are disclosed in particular by Patent EP 803,925.
PAN, which corresponds to the general formula xe2x80x94[CH2xe2x80x94CH (CN)]n, is an industrial polymer used as textile fiber or as precursor for carbon fibers. It comprises a nitrile group (CN) which provides good interaction with the lithium cation (Li+). This polymer is disclosed in particular by Patents WO 95/02314 and U.S. Pat. No. 5,219,679 and partially by EP 803,925. However, this polymer has disadvantages.
Thus, when it is used in combination with a metallic lithium electrode, the interfacial resistance between lithium and electrolyte steadily increases with contact time.
This disadvantage is partly related to the presence, in the repeat unit, of a hydrogen carried by the tertiary carbon which is situated in the a position with respect to the nitrile group (CN). The highly electron-withdrawing effect of this nitrile group renders this hydrogen markedly more acidic. The result of this is amodification over time in the performance of the generator. The interfacial resistance gradually increases, which affects the performance of the battery, in particular the power delivered. Furthermore, thermal degradation of the PAN is reflected by the release of hydrocyanic acid (HCN), which is harmful from the viewpoint of safety of use, in particular for use by the general public.
Finally, from the process viewpoint, PAN is not soluble in solvents with low boiling points. The most commonly used solvent for PAN is DMF or dimethylformamide, the boiling point of which is 152xc2x0 C. Such a characteristic excludes certain processes for shaping the polymer electrolyte.
This is because one of the conventional routes for the preparation of polymer electrolytes for lithium batteries consists in dissolving a polymer, a salt and a mixture of constituent solvents of the polymer electrolyte in a common solvent, preferably with a low boiling point, and then, after having cast the mixture in the form of a film, in driving off the common solvent at atmospheric pressure or under reduced pressure without removing the constituent solvents of the plasticized polymer electrolyte. The use of a common solvent such as DMF excludes a preparation process incorporating solvents with boiling points lower than that of DMF. In other processes, the PAN+ salt mixture is dissolved under warm conditions in a mixture of cyclic carbonates, before being cast in order to produce a polymer electrolyte membrane. Dissolution is carried out at an excessively high temperature which excludes, for example, the use of noncyclic carbonates.
The aim of the invention is to provide a solid polymer electrolyte which does not exhibit such disadvantages. A polymer electrolyte according to the invention is less reactive with respect to lithium than PVDF. It is more stable from the electrochemical viewpoint than PAN and less toxic than the latter from the viewpoint of the emission of hydrocyanic acid (HCN) following thermal degradation. Furthermore, the preparation of the polymer electrolyte according to the invention is simpler because, unlike PVDF and PAN, the polymer which it employs is soluble in many conventional solvents with low boiling points, such as acetone.
The solid polymer electrolyte according to the invention comprises a polymer and a salt, which may or may not be attached to the polymer, and it is characterized in that the polymer is a methacrylonitrile polymer in the form:
of a linear homopolymer of high mass, which may or may not be reinforced, or
of a homopolymer, which may or may not be reinforced, rendered three-dimensional by crosslinking, or
of a linear copolymer of high mass or of a copolymer rendered three-dimensional by crosslinking, by virtue in particular of the incorporation of at least one comonomer which makes crosslinking possible.
An electrolyte of the invention can optionally comprise, in addition, at least one aprotic liquid solvent (which will generally have a plasticizing effect) and optionally a filler.